Pressure ulcers result from excessive pressure applied to fragile tissue over an extended period of time. The wounds usually occur over bony prominences where weight bearing soft tissue is compressed. The most common anatomical locations are the sacrum, heels and ischial tuberocities. Known risk factors include pressure, shear force, heat, moisture, aging skin, immobility, lack of sensation, poor nutrition, diabetes and a host of other conditions. The cause is believed to be ischemia, ischemia-reperfusion injury, or simply mechanical damage due to cell deformation. Populations at high risk include the immobile elderly, persons with paralysis and/or neurological deficits, and people undergoing long surgical procedures. Primary prevention involves frequent repositioning and the provision of a cushioning surface that redistributes pressure away from vulnerable areas.
Fifteen years ago, a group of researchers at the University of Minnesota (Kokate et al., 1995) demonstrated the critical importance of skin temperature to the susceptibility of tissue to ulceration when the tissue is exposed to ischemia inducing compressive forces. In an experiment on the backs of swine, pressure (100 mmHg) was selectively applied with skin temperature controlled at 25° C., 35° C., 40° C. and 45° C. for five hour periods. Histological examination at 7 days post-procedure showed that all tissue layers were normal under 25° C., while moderate muscle damage was found at sites with 35° C., and tissue necrosis at all layers was found at sites with 45° C. In a follow up study, Iaizzo (1995) narrowed the temperature range to 25° C., 27° C., 30° C. and 32° C. in an attempt to find a critical temperature. The results were not conclusive, but based on the relationship between temperature and damage severity that they observed, they suggested temperatures below 30° C. had a protective effect for the conditions of the experiment. The fundamental effect of the cooling is believed to be a lowering of the metabolic rate (consumption of O2) in ischemic tissue allowing the cells to survive for a longer period of time without oxygen.
Despite Kokate and Iaizzo's demonstration of the prophylactic effect of maintaining skin temperature at levels below core body temperature and below typical skin temperature, no cushions, mattresses, operating table overlays, or other cushioning surfaces have been brought to market that actively cool the skin in an attempt to prevent pressure ulcers. Possible explanations for this are numerous, but chief among them is the undesirable effect of lowering core body temperature in an attempt to cool the skin impacted by weight bearing. A potential solution is to limit the location where cooling is applied to only those locations where pressure exceeds a given threshold, thus limiting the overall effect on the body while cooling the most critical areas. Iaizzo and others, e.g., Augustine, et al., U.S. Pat. No. 6,497,720, teach the concept of applying active cooling in response to pressure. Such pressure sensitive cooling techniques for cushioning are complex. A simpler approach according to preferred embodiments of the present disclosure comprises an actively and selectively cooled cushioning surface with embedded cooling mechanisms that cool tissue that reaches a predefined level of immersion into the cushioning surface (i.e., immersion-based cooling).